1. Field of the Invention
The present invention is directed to the manufacture of electronic devices, and in particular electronic devices formed using one or more thin flexible sheets of an inorganic material.
2. Description of Related Art
Display devices utilizing plasma, liquid crystal, or organic light emitting diode display elements, to name a few, are fast overcoming cathode ray tube (CRT) displays in commercial products, finding use in a myriad of applications, from cell phones to televisions. However, the introduction of very thin, light weight, or flexible displays is only in its infancy. This is due in no small part to the tremendous structural demands placed on such display devices: they must be capable of withstanding repeated flexing or bending or other stress without harm to the device or the substrate on which it is disposed; due to the intended use of thin, light, or flexible displays in portable devices, they are expected to withstand rough handling, again without undue harm to the device or substrate, and; they must be capable of withstanding impact or a bending radius that can be less than 2 cm, and less than 1 cm in some cases.
One material contemplated for use in thin, light weight, or flexible displays or electronic devices is glass. Glass is generally chemically resistant, transparent, can form a hermetic barrier or seal, can tolerate typical electronic fabrication temperatures, and may be formed into very thin sheets. Sheets in excess of 10 m2 having thicknesses less than 1 mm, and even less than 0.7 mm have been produced and routinely used, and glass sheets are soon expected to reach dimension of at least about 100 m2. In a typical display manufacturing process, multiple displays are formed using one or more large glass sheets or substrates. The displays are then separated into individual display units, usually by scoring and breaking or other cutting methods. Thus, very large glass sheets are efficiently utilized by producing as many display or electronic units as possible.
Cutting glass, and in this case glass sheets, generally forms flaws (e.g. cracks) in the edges of the glass sheets. These flaws can serve as crack initiation sites, and thereby reduce the strength of the sheets, particularly if the glass is flexed such that the flaw experiences tensile stress. Generally, typical display devices do not experience significant flexing, thus the existence of these flaws is not of significant concern: Typical cutting methods produce edges of sufficient strength to survive both the standard device processing conditions and current application end use.
Shown in FIG. 1 is a Weibull plot showing the failure probability for 75 micron thick glass sheets in four-point bending according to standardized four-point bending tests (e.g. ASTM). The samples in this case were 5 mm wide×30 mm long×75 microns thick. The samples were tested in a four-point bend arrangement standing on edge so the tensile stress was applied across the entire 75 um face thickness. The glass sheet represented by curves 10 and 12 were laser cut, while the glass sheet represented by curve 14 was mechanically scribed and separated by bending to fracture the sheet. As depicted, none of the samples represented by the curves showed a high probability of withstanding a stress in excess of about 300 MPa. The samples for mechanical scoring, the most widespread method of separating glass, did not show a high probability of withstanding a stress in excess of 100 MPa. Although standard cutting methods for glass substrates greater than 0.4 mm thick address the needs of current device manufacturing processes or application end use, higher edge strength is required for substrates less than 0.4 mm thick as may be used in emerging processes and applications such as flexible displays.
Flexible displays or flexible electronic devices, by the very nature of their flexibility, may produce significant stress in the display or electronic substrate(s), either during the manufacturing process or in use. Thus, flaws that might be present in the glass may experience stresses sufficiently great that the glass will crack, causing the glass to fail. Since typical display manufacturing involves cutting the glass to form individual displays, and cutting is known to create multiple flaws in the glass along the cut edge, this bodes poorly for the fate of glass substrate-based flexible display devices.
Attempts to mitigate flaws at the edges of glass sheets have included laser cutting, grinding, polishing and so forth, all in the attempt to remove or minimize the flaws that are created when the glass sheet is cut to size. However, many of these approaches are unsatisfactory for flexible electronic applications, either because the technique is incapable of removing flaws down to the size needed for the expected stresses, or the technique is difficult to apply to such thin glass sheets (less than about 0.4 mm thick) in a manufacturable process or scale. Acid etching of the glass edges may be used, but acid etching may also degrade the display or electronic device disposed on the substrate.